Recently, the requirement for the quality of hydrocarbon products has become more and more critical with the growing tendency for raw oil to become heavier and the increase of the consideration for environmental protection. In 1980's, the progress for preparing lead-free gasoline promoted the development of the catalytic cracking process and the catalysts. The Y-zeolite with high silica content as well as various catalysts and auxiliaries with high octane number were produced and used in the conversion of cheap heavy oil to the light products with high additional values.
The processing of inferior residuum has been long concerned with in petroleum refining industry. It is an important problem need to be resolved urgently that how to obtain fuel with high yield of light oil, less olefin and sulfur, as well as being environmentally friendly, when treating the inferior residuum and blending more residuum to the feedstock oil.
In U.S. Pat. No. 3,293,192 and Society of Chemical Engineering (London) Monograph Molecular Sieves, P. 186 (1968), C. V. M C. Daniel and P. K. Maher reported a hydrothermal dealumination method used to prepare Y-zeolite with high silica content, since then the method has been widely used. U.S. Pat. No. 3,442,715 disclosed a dealuminated Y-zeolite (DAY), U.S. Pat. No. 3,449,070 disclosed an ultra-stable Y-zeolite (USY), U.S. Pat. No. 4,51,694 and U.S. Pat. No. 4,401,556 disclosed an ultra-stable hydrophobic Y-zeolite, etc. The ultra-stable Y-zeolite (USY) was widely used and developed because it has high Si/Al ratio, high hydrothermal stability, low hydrogen transfer activity and good selectivity of coke and can be used to prepare catalyst with high octane number when treating heavy oil.
With the increasing development of heavy oil processing, the characters of ultra-stable Y-zeolite with high silica content, such as high Si/Al ratio, high hydrothermal stability and low hydrogen transfer activity, are no more suitable for the worse of the inferior heavy oil and cannot satisfy the environmental requirement on catalytic cracking cleaning fuel. It's urgent to find out a kind of zeolite, which has high hydrothermal stability, high cracking activity, good hydrogen transfer activity, stability and selectivity of coke, and the ability to decrease the olefin and sulfur content of gasoline effectively while increasing the conversion of heavy oil.
In order to increase the activity of Y-zeolite with high silica content, U.S. Pat. No. 4,840,724 discloses a method to increase the rare-earth content of ultra-stable Y-zeolite by rare-earth ion exchange. The process used to prepare Y-zeolite with high silica content (USY) through hydrothermal method has many hydrothermal exchange and high-temperature calcination procedures. In the process of dealumination with supplements of silica, a lot of dealuminated holes are formed and cannot be filled in time with the silica migrating from the skeleton, which usually leads to the collapse of the crystal lattice of the zeolite. The resulted silica and alumina fragments tend to block the channels of the zeolite and are not easily eliminated, which results in the low ion exchange level of the rare-earth. In the rare-earth ultra-stable Y-zeolite (REUSY), the highest RE2O3 content, used to express the rare-earth content, is only about 3% by weight after many times of ion exchange and calcinations. Such a low RE2O3 content is far from satisfying the requirement of high cracking activity and stable hydrogen transfer activity for the catalyst in the deep treating of inferior raw oil.
In order to increase the activity of Y-zeolite and utilize the high ion exchange capacity of NaY zeolite, REHY and REY zeolites with high rare-earth content through are prepared many times of rare-earth ion exchange and high-temperature calcination procedures to increase the rare-earth content of the zeolite. The RE2O3 content of REHY zeolite was increased to about 6˜16% by weight and the RE2O3 content of REY zeolite was increased to about 8˜20% by weight, which may satisfy the requirement of increasing the hydrogen transfer activity of the zeolite and thereby achieve the object of increasing cracking activity,
But the use of hydrothermal method with many times of ion ex-change between ammonium ion and rare-earth ion and high-temperature calcination usually leads to the collapse of the crystal lattice of REY and REHY zeolite. The resulted silica and alumina fragments tend to block the channels of the zeolite and are not easily eliminated, which in turn inhibit the effective migration of rare-earth ion (RE3+) to β-cage of the zeolite, REHY and REY zeolites have a fatal defect in that the original crystal cell is hard to shrink, the Si/Al ratio is low, the unit cell size is of 2.470 nm and the zeolite skeleton structure is not stable. As a result, the prepared catalyst shows high original cracking activity and hydrogen transfer activity, is easy to be deactivated in reaction equipments, and the hydrothermal stability, the equilibrium activity as well as the equilibrium unit cell size of Y-zeolite used as active component are low. During manufacture process, a large amount of fresh catalyst need to be added to keep activity because of the rapid decrease of the activity, the lost of the original hydrogen transfer activity and the decrease of the coke selectivity of the catalyst. In the manufacture of hydrocarbons the low stability of cracking activity and hydrogen transfer activity of the catalyst results in a high consumption of the catalyst, a high product yield of coke and an unstable operation of the equipment. So they cannot be widely used in the deep treating of inferior raw oil and fulfil the purpose of manufacturing environmentally friendly fuel and decreasing the olefin content of gasoline.